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does LLC miss equals memory access ?

li_d_
Beginner
171 Views

Hi, all

I'm new in computer architecture, I have a question may look silly.

I use lmbench's bw_mem tool to test mem bandwidth. my testbed is a 4 sockets(E7-4870) system populated 64 8GB dimms.

Using numactl to bind node 0 read node 1's memory. the test result is 11GB/s. And I using PCM to monitor CPU activities. Found L3MISS is 88M and MC READ is 12.53GB.

IMHO the llc misses should equals to memory access, so 64B(cache line size) * L3miss should equal to mem bandwidth. But the test result is 64B * 88M = 5.632GB << 12.53GB, what am i missing here?

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3 Replies
Bernard
Black Belt
171 Views

Do you mean total memory bandwidth?It could also contain prefetched accesses and cache hits

Patrick_F_Intel1
Employee
171 Views

Hello Li,

I'm guessing that the hardware prefetcher is pulling the data into the L3 before the cacheline is accessed. So the L3 doesn't see the 'miss'. This is the purpose of the prefetchers.

The lmbench rd benchmark reads every 4th integer. If it read just 1 integer per cacheline (and if the generated assembly code is effiicent) then you might get more L3 misses.

Pat

li_d_
Beginner
171 Views

thank you all, I understand LLC miss and memory access more now.

I turn the prefetchers off the result seems more resonable.  On another 2-sockets system  LLC miss is 145M, MC READ is 8.66GB.  145M*64 = 9280M

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